1. Field of the Invention
This invention relates generally to a method for the control of oil and gas production wells. More particularly, it relates to proportional control of movable elements in well production flow control valves.
2. Description of the Related Art
The control of oil and gas production wells constitutes an on-going concern of the petroleum industry due, in part, to the enormous monetary expense involved in addition to the risks associated with environmental and safety issues. Production well control has become particularly important and more complex in view of the industry wide recognition that wells having multiple branches (i.e., multilateral wells) will be increasingly important and commonplace. Such multilateral wells include discrete production zones which produce fluid in either common or discrete production tubing. In either case, there is a need for controlling zone production, isolating specific zones and otherwise monitoring each zone in a particular well. Flow control devices such as sliding sleeve valves, downhole safety valves, and downhole chokes are commonly used to control flow between the production tubing and the casing annulus. Such devices are used for zonal isolation, selective production, flow shut-off, commingling production, and transient testing.
These tools are typically actuated by hydraulic systems or electric motors driving a member axially with respect to a tool housing. Hydraulic actuation can be implemented with a shifting tool lowered into the tool on a wireline or by running hydraulic lines from the surface to the downhole tool. Electric motor driven actuators may be used in intelligent completion systems controlled from the surface or using downhole controllers.
The surface controllers are often hardwired to downhole sensors which transmit information to the surface such as pressure, temperature and flow. It is also desirable to know the position of the movable members, such as, for example, the sliding sleeve in a sliding sleeve valve, in order to better control the flow from various zones. Originally, sliding sleeves were actuated to either a fully open or fully closed position. To control an open-closed, hydraulically actuated flow control device, it is sufficient to provide a simple open loop control system. The principal problem with this arrangement is that there is no way to confirm that the device has actually performed the desired action. To obviate this problem, sensors are placed downhole to directly sense the position of the device.
To implement a valve with proportional control, a closed loop feedback control system is used. The proportional control allows the valve to function in a choking mode which is desirable when attempting to commingle multiple producing zones that operate at different reservoir pressures. This choking prevents crossflow, via the wellbore, between downhole producing zones. The closed loop system typically requires sensors and control system electronics to be mounted downhole. However, the combination of high pressure and high temperature act to reduce the effective lifespan of the downhole electronics and reduce the reliability of the overall system. It is highly desirable to reduce or eliminate the complex system of downhole sensors and electronics.
What is desired is a simple proportional control system. An obvious solution is the use of an open-loop control system. This would be possible if the controlled devices and sensors did not degrade and change with time. In the case of a hydraulically powered sliding sleeve valve, the valve experiences several changes over time. For example, hydraulic fluid ages and exhibits reduced lubricity with exposure to high temperature; scale and other deposits will occur in the interior of the valve; and seals will degrade and wear with time. For a valve to act effectively as a choke, it needs a reasonably fine level of controllability. The potential changes to the system components prevent that controllability with an open-loop control system.
Thus there is a need for a simple proportional hydraulic actuation system for downhole flow control devices which can determine the position of a downhole movable member using surface located control and feedback components. The system must be able to adapt to and compensate for the exposure related changes to the downhole system.
The methods and apparatus of the present invention overcome the foregoing disadvantages of the prior art by providing a system and methods for effecting the simplicity of an open loop control system and the controllability of a closed loop system by adaptively determining system response changes over time using surface located sensors and controlling proportional valve movement based on the revised system response.
The present invention contemplates a surface located system and sensors for deriving appropriate feedback control parameters to effect proportional control of a downhole hydraulically actuated flow control device.
In one preferred embodiment, a system for controlling a downhole flow control device comprises an hydraulically actuated flow control device in a production string. The flow control device has a movable element for controlling the downhole formation flow. A hydraulic system is hydraulically coupled to the hydraulically actuated flow control device and supplies hydraulic fluid to the hydraulically actuated flow control device. At least one sensor detects at least one parameter of interest related to a volume of hydraulic fluid supplied to the hydraulically actuated flow control device and generates a first signal related thereto. At least one pressure sensor determines a hydraulic fluid supply pressure and generates a second signal related thereto. A processor receives the first signal and the second signal and acts according to programmed instructions to generate a relationship between a position of the moveable element and the volume of supplied hydraulic fluid and controls the hydraulic system to position the spool at a predetermined position according to the relationship.
In a second preferred embodiment, a system for controlling a downhole flow control device comprises an hydraulically actuated flow control device in a production string, where the flow control device has a movable element for controlling the downhole formation flow. A hydraulic system is hydraulically coupled to the hydraulically actuated flow control device for supplying hydraulic fluid to the hydraulically actuated flow control device. At least one sensor detects at least one parameter of interest related to a volume of hydraulic fluid supplied to the hydraulically actuated flow control device and generates at least one first signal related thereto. At least one pressure sensor for determining a hydraulic fluid supply pressure and generating at least one second signal related thereto. A hydrophone disposed in a hydraulic line detects a pressure pulse in response to movement of the spool and generates a third signal in response thereto. A processor receiving said first signal, said second signal, and said third signal and acting according to programmed instructions to generate a relationship between a position of the spool and the volume of supplied hydraulic fluid and controls the hydraulic system to position the moveable element at a predetermined position according to the relationship.
In another preferred embodiment, a method for control of a hydraulically actuated, downhole flow control device comprises cycling a moveable element in the hydraulically actuated downhole flow control device in a first direction and a second opposite direction. A breakout pressure is determined for each actuation cycle using a pressure sensor. The device is cycled until the breakout pressure on successive cycles is within a predetermined difference while measuring the pumped fluid volume for each cycle or until a predetermined number of cycles have occurred. A processor generates a relationship characterizing the movement of the moveable element as a function of a pumped fluid volume and controls the supply of fluid required according to said relationship to move said moveable element to a predetermined position.
In another preferred embodiment, a method for control of a hydraulically actuated, downhole flow control device comprises cycling a moveable element in the hydraulically actuated downhole flow control device in a first direction and a second opposite direction. A breakout pressure is determined for each actuation cycle using a pressure sensor and a hydrophone. The device is cycled until the breakout pressure on successive cycles is within a predetermined difference while measuring the pumped fluid volume for each cycle. A processor generates a relationship characterizing the movement of the spool as a function of a pumped fluid volume and controls the supply of fluid required according to said relationship to move said moveable element to a predetermined position.
In another preferred embodiment, a method for proportional control of a hydraulically actuated, downhole flow control device, comprises supplying hydraulic fluid to an actuator cooperatively coupled to a spool in the hydraulically actuated downhole flow control device through a first line and a second line. The first line and the second line are pressured to the same predetermined pressure. A first measured volume of hydraulic fluid is bled from the second line causing the actuator to move the spool. A second measured volume of hydraulic fluid is supplied to the first line until the first line is at the predetermined pressure. A volume difference is determined between the second measured volume and the first measured volume. A surface located processor is used to generate a relationship between the volume difference and the spool movement. The relationship is used to move said moveable element to a predetermined position.
In another preferred embodiment, a method for proportional control of a hydraulically actuated, downhole flow control device, comprises supplying hydraulic fluid to an actuator cooperatively coupled to a spool in the hydraulically actuated downhole flow control device through a first line and a second line. The first line and the second line are pressured to the same predetermined pressure. A first measured volume of hydraulic fluid is bled from the second line causing the actuator to move the spool. A second measured volume of hydraulic fluid is supplied to the first line until the second line is at the predetermined pressure. The first line pressure is then adjusted to the predetermined pressure and a volume difference is determined between the second measured volume and the first measured volume. A surface located processor is used to generate a relationship between the volume difference and the spool movement. The relationship is used to move said moveable element to a predetermined position.
Examples of the more important features of the invention thus have been summarized rather broadly in order that the detailed description thereof that follows may be better understood, and in order that the contributions to the art may be appreciated. There are, of course, additional features of the invention that will be described hereinafter and which will form the subject of the claims appended hereto.